In Genesis 1:6, the word "firmament" (Hebrew: raqiya) is referring to what we call the sky. It separates the waters below (oceans, lakes, rivers, etc) from the waters above (where the rain comes from). Just two verses later, we see that God calls the firmament shamayim, which is to say the visible heavens or "sky". Strong's suggests the word means something akin to a vaulted ceiling. However, one can can see the same word raqiya used in Dan 12:3 where it obviously does indicate the sky.
So, why does the text seem to indicate some kind of support for the rain or a background to which the stars are pasted? I don't believe the Hebrews considered the sky to be a solid thing holding up the waters (or the stars). At least not as such. I contend that sometimes the language in the Bible is poetic and we must be careful to not to take things literally that are not meant to be taken that way. Today we might poetically compare the sky to the ceiling of a cathedral. They did not have cathedrals 3,500 years ago but palaces or temples with vaulted ceilings were known to Sumerian/Babylonian and Egyptian architecture so is it likely the original audience would have understood the image that God was using perhaps to inspire to His people who had just left Egypt (and may have built some of those same type ceilings)? Or did they truly believe a fixed, solid expanse existed and God simply spoke to them in the vernacular of their day?